I've always used the API or Javascript for this function and haven't been able to find much on use cases and application for the Formula field.

Do you happen to have a resource that shows 'Field Math' using a formula field? You seem to really know the platform so I'm hoping you have some insight. Is this something most people just do in SalesForce?

The example in the product doc is not quite the same as doing a calculation. It is more determining a static value based on another field's value.

my assumption was wrong. It is best to run formulas in SFDC instead or with a webhook or something else. I've run into some challenges where the SFDC formula field might not show up right in Marketo, but I think that was only with a lookup field.